1. Field of the Invention
The present invention relates to a writing control device and an image forming apparatus, for dispersedly storing data in a plurality of HDDs.
2. Description of Related Art
Conventionally, in an image forming apparatus, an HDD (Hard Disk Drive) is used as a medium for storing a large-volume data, such as an image data. The HDD is configured so as to contain a disk for storing data in a casing. In the HDD, by rotating the disk and seeking a specific sector with a head, a data is written or read.
In many cases, the data are written in order from the tracks of the outer peripheral portion toward the tracks of the inner peripheral portion on the disk (platter). FIG. 7 shows the situation in which the data are written in one HDD in order from the tracks of the outer peripheral portion toward the tracks of the inner peripheral portion. In the HDD, the outer peripheral portion is different from the inner peripheral portion in the size of data which is written or read during one rotation of the disk. As the tracking proceeds to the inner side, the data transfer speed is low (the performance of the HDD is deteriorated).
For example, the graph 90 of FIG. 8 shows the relation between the position (track) on which the data is written in or read from the HDD, and the data transfer speed. In the graph 90, the abscissa axis indicates the track and the ordinate axis indicates the data transfer speed. As the position (track) on which the data is written or read proceeds to the inner side, the data transfer speed becomes low.
The graph 91 of FIG. 9 corresponds to the graph 90 of FIG. 8, and shows the relation between the position (track) on which the data is written in or read from the HDD, and the time which is necessary to transfer the data having the predetermined size. In the graph 91, the abscissa axis indicates the track and the ordinate axis indicates the time which is necessary to transfer the data. The time which is necessary to transfer the data can be calculated by dividing the size of data to be written or read by the data transfer speed. Because the data transfer speed becomes low as the position (track) on which the data is written or read proceeds to the inner side, the time which is necessary to transfer the data is increased.
In Japanese Patent Application Publication No. 2008-286756, the following technology is disclosed. In the technology, in order to close the difference in the performance (data transfer speed) between the outer peripheral portion and the inner peripheral portion, when the data is stored, the highly-compressed data is written in the outer peripheral portion in which the data is written or read at a high speed, and the low-compressed data is written in the inner peripheral portion in which the data is written or read at a low speed (the compression ratio of the data to be written is changed depending on the outer peripheral portion or the inner peripheral portion). As a result, the time which is necessary to expand the read data becomes constant.
The data transfer speed of one HDD has an upper limit. Therefore, there are some cases in which the performance which is necessary for an image forming apparatus, cannot be obtained by the data transfer speed of one HDD.
For example, in FIG. 8, even the data transfer speed in the track of the outermost periphery is less than the speed which is necessary for an image forming apparatus to maintain the productivity (hereinafter, referred to as “productivity maintaining speed”). Further, in FIG. 9, even in the track of the outermost periphery, the time which is necessary to transfer the data is more than the time limit for satisfying the productivity in the image forming apparatus. That is, the performance which is necessary for the image forming apparatus to maintain the productivity is not obtained.
As a method for increasing the data transfer speed, a method for storing data by using the RAID (Redundant Arrays of Inexpensive Disks) structure in which a plurality of HDDs are provided (RAID 0 (striping)), has been proposed. In the striping, the data to be stored is equally divided into the number of HDDs, and is dispersedly stored. When the data is written by the striping in a plurality of HDDs which constitute the RAID, the division data into which the data is divided are stored in the tracks (sector) having the identical address, respectively.
FIG. 10 shows the situation in which the data is dispersedly stored by the striping in three HDDs. Also, when the striping is carried out, the division data are written in order from the tracks of the outer peripheral portion toward the tracks of the inner peripheral portion. However, in the RAID 0 (striping), the data transfer speed of the whole RAID 0 is calculated by multiplying the lowest data transfer speed among a plurality of HDDs which are activated in parallel, by the number of HDDs. Therefore, when the data transfer speed of only one HDD is decreased, the data transfer speed of the whole RAID 0 is largely decreased with decreasing the data transfer speed of the HDD. In case that the data is written in or read from the outer peripheral portion of the disk, the desired data transfer speed is achieved. On the other hand, in case that the data is written in or read from the inner peripheral portion of the disk, the desired data transfer speed could not be achieved.
The graph 92 of FIG. 11 shows the relation between the position (track) on which the data is written in or read from the HDD, and the time which is necessary to transfer the data, in case that the striping is carried out by using three HDDs. In the graph 92, the abscissa axis indicates the track and the ordinate axis indicates the time which is necessary to transfer the data. In FIG. 11, the time which is necessary to transfer the data in case that each of the above three HDDs is used alone, is the same as the time shown in the graph 91 of FIG. 9.
In the graph 92 of FIG. 11, like the graph 91 of FIG. 9, as the position (track) on which the data is written or read proceeds to the inner side, the time which is necessary to transfer the data is increased. However, because the data is equally divided into three division data and the division data are written in or read from three HDDs in parallel, the data transfer speed of the whole RAID is high. As compared with the graph 91 of FIG. 9, the time which is necessary to transfer the data is short.
In the graph 92, from the track of the outermost periphery to the track of T1, the time which is necessary to transfer the data is not more than the time limit for satisfying the productivity in the image forming apparatus. As a result, the performance which is necessary for the image forming apparatus to maintain the productivity is obtained (the data transfer speed of the whole RAID is not less than the productivity maintaining speed).
However, in the tracks of the inner peripheral portion which is an inner portion from the track of T1, even though the striping is carried out, the time which is necessary to transfer the data is over the time limit for satisfying the productivity in the image forming apparatus (the data transfer speed of the whole RAID is less than the productivity maintaining speed). The performance which is necessary for the image forming apparatus to maintain the productivity is not obtained. Therefore, in a conventional technology, the tracks of the inner peripheral portion which is an inner portion from the track of T1 in the HDD, is set to the write inhibit area, and the data is written in or read from only the tracks of the outer peripheral portion which is an outer portion from the track of T1. As a result, the desired data transfer speed is secured and the performance which is necessary for the image forming apparatus to maintain the productivity is achieved. However, when the HDD is used as described above, the problem in which the inner peripheral portion of the HDD is not used is caused.
By increasing the HDDs which constitute the RAID, it is possible to reduce the write inhibit area. However, the cost and the space of the HDDs to be increased are required.
In the method disclosed in Japanese Patent Application Publication No. 2008-286756, in case that the data is stored in one HDD, the compression ratio of the data is changed depending on the position to which the data is written. The disclosed method is not available for the case in which a plurality of HDDs are used or the case in which the data is divided.